Discrepant analysis and screening for Chlamydia trachomatis

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Sir—McGee and Reynolds are correct to note that upwardly biased estimates of sensitivity and specificity could have “profound significance for medicine”. Even a 1% decrease in specificity will result in a substantial amount of unnecessary treatment and follow-up of false-positive individuals. In urine from females without symptoms the specificity of LCR is 95·2%, according to the Food and Drug Administration-approved package insert. With discrepant analysis by the MOMP gene test specificity would have been put at over 99·5%, a difference that has serious implications for screening, so Schachter and colleagues are wrong to imply that my article is “much ado about very little”. Discrepant analysis and screening for Chlamydia trachomatisHadgu (Aug 31, p 592)1 points out that discrepant analysis as a method for determining the specificity and sensitivity of a screening or diagnostic test leads to biased and misleading estimates. These biases have a profound significance for medicine. If we use the estimates in table 2 of Hadgu's paper (sensitivity 94·4%, specificity 99·9%), the predictive value positive of the Chlamydia trachomatis test used in a family planning clinic where the true prevalence is 2% is calculated as over 95%. If, however, the true sensitivity and specificity were 91% and 97%, the predictive value positive decreases to 38%. Full-Text PDF Discrepant analysis and screening for Chlamydia trachomatisHadgu's article on discrepant analysis1 for the evaluation of new methods for diagnosing genital infection by Chlamydia trachomatis makes use of results obtained by DNA amplification (ligase chain [LCR] or polymerase chain [PCR] reactions) in comparison with cell culture. Discordant results are usually resolved by additional methods. This may, according to Hadgu, introduce bias in favour of the new test. The article is timely. We accept his analysis and his statement about the risk of overestimating the sensitivity of the new tests (LCR, PCR) to the disadvantage of cell culture (94·4% vs 65·0% in the article Hadgu refers to2). Full-Text PDF Discrepant analysis and screening for Chlamydia trachomatisHadgu raises concerns about how diagnostic tests for Chlamydia trachomatis are evaluated.1 He alleges that use of discrepant analysis strongly biases these evaluations in favour of new tests, with negative connotations for standard tests. We think that these concerns are “much ado about very little”. We and others have used discrepant analysis with DNA amplification tests for C trachomatis because this approach provides the most accurate and practical means of understanding the performance of these new and highly sensitive assays. Full-Text PDF